Welcome to UX.SE @Florent2. The best way to get answers here are if you're question can be answered - and that their is a correct answer to give. Asking for "example of..." is not a very good format for a question at a Q&A. However, I thonk you can rephrase the question, making it answerable, cause I've never heard of onboarding before. Maybe ask "What is onboarding?" or "How do I use onboarding?" - and I'm sure you'll get good answers. Nice to see you here!
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Benny Skogberg♦Oct 19 '12 at 6:36

I think by onboarding he means the first few interactions you have with a site, that help you orient yourself. Analogous to this cgcareers.org/articles/detail/1014 but I'm not going to swear to it, because until about 2 minutes ago I'd never heard the word! If that's right other terms might be learnability or training-wheels. So a better question might be "what features and guidance would make a website quick to learn to use?"
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Peter BagnallOct 20 '12 at 0:28

There are various approaches like Slideshows, ToDo-lists (achievements) or guided tours.

The one to chose depends on how your website (i think you really mean 'web app' as websites should never need onboarding, IMHO) is organized and what you expect the users to to with it.

If it is a highly interactive app with a lot of tools where users can create things (e.g. drawing) its best to have a guided tour or an interactive tutorial. Giving the user a task and showing him how to archive that task. (Task: "Draw a circle in this area"). This technique is rather time consuming to implement. But promises the best learning and user connection result.

If your app is more focused on consumption (e.g. feed reader) an annotated screenshot might be enough.

http://www.seomoz.org - takes you through a 'wizard' that sets you up with each of the tools in their SEO toolset explaining clearly and with examples what each does and what you need to do to make the most of it.

Although the interface on site isn't that stellar; http://www.seobook.com - has as well through out series of emails post sign-up to drive usage and explain individual features from their toolset.

I don't know which devices you are taking into account when considering the onboarding process, however I recently used M.DOT to create a mobile site and thought they did a fantastic job of designing the signup/profile creation process.

I set up my account and immediately was given the opportunity to design the site directly from my phone(something I would have thought difficult considering the screen size).

Might be it is too late to answer. But this may be helpful to other users who bounce to this. Onborading is one of the interesting parts in UX. A right blend of minimal design +trust + value selling will make user onboarding delightful. I have not explained the above in detail because depending upon the genre of website and audience the onboarding varies. Rather see this website http://www.useronboard.com where various generes of website onboarding is critiqued. This will help you to do an analysis for the website that you develop.